It is a processing technique which is widely carried out that two plates are overlapped with each other, an edge of one plate is bent on an edge of the other plate, and the edges of the two plates are connected with each other. This technique is called hemming.
Although a press hemming method using a press die is known well as the hemming, a roller hemming method that is another method is also adopted (for example, refer to JP-A-07-016731 (refer to FIGS. 1 and 3)).
JP-B-07-016731 is described on the basis of the following drawings. FIGS. 16(a) and 16(b) are views illustrating a basic principle of a conventional technique. Referring to FIG. 16(a), a workpiece 102 is placed on a hemming die 101, and an edge of the workpiece 102 is pressed down by clamping arms 104 and 104 of clamping mechanisms 103 and 103.
Referring to FIG. 16(b), the workpiece 102 is obtained by overlapping a second plate 108 on a first plate 107 having an erected flange 106. Then, the erected flange 106 is bent by a roller 111 provided in a robot arm 110. As a result, the edge (erected flange 106) of the first plate 107 can be wound around the edge of the second plate 108, and connection between the first plate 107 and the second plate 108 can be made.
FIG. 17 is a sectional view taken along the line 17-17 of FIG. 16(b), and shows an example in which the workpiece 102 is pressed down by a plurality of clamping arms 104A, 104B, 104C, and 104D. Meanwhile, it is necessary to make a shape of a receiving face 113 of the hemming die 101 to be the same as a shape of the first plate 107. Since the hemming die 101 is a machined part, its accuracy is excellent. On the other hand, the shape of the first plate 107 that is obtained by plastically deforming a blank material varies under the influence of springback, etc.
When the clamping arms 104A, 104B, 104C, and 104D are in an unclamping state, a gap 114 is formed locally. If the clamping arms 104A, 104B, 104C, and 104D are turned into a clamping state, the gap 114 can be made substantially zero, especially by the operation of the clamping arms 104C and 104D.
If the workpiece 102 is a so-called single article, such as a bonnet or a door, the gap 114 can be eliminated by making an arrangement and number of clamping arms proper. In addition, the single article refers to articles of a size to be placed on the hemming die 101.
Next, a case where a workpiece is larger than the single article, that is, a large-sized part that protrudes largely from a hemming die will be discussed. FIGS. 18(a) to 18(d) are explanatory views when a conventional roller hemming method is applied to a vehicle body. Although the schematic view of the workpiece 120 is shown in FIG. 18(a), the workpiece 120 is a white body and is a large-sized structure that is obtained by combining a plurality of panels. FIG. 18(b) is an enlarged view of a section B of FIG. 18(a), and shows a rear wheel arch 121.
FIG. 18(c) is a sectional view taken along the line C-C of FIG. 18(b), and shows a state before a second plate 124 is overlapped on a first plate 123 having an erected flange 122, and the erected flange 122 is bent by hemming. FIG. 18(d) shows a state after hemming, and shows that the second plate 124 can be combined with the first plate 123 by winding a lower end 122, i.e., an erected flange, of the first plate 123, into a lower end of the second plate 124.
FIG. 19 illustrates a problem when a conventional roller hemming method is applied to a vehicle body, and shows a posture in which the first plate 123 is placed on the hemming die 101, and the erected flange 122 is bent onto the second plate 124 placed on the first plate 123, using a roller 106.
At this time, although the rear wheel arch 121 is pressed down by the clamping arms 104A, 104B, 104C, and 104D, the first plate 123 is a rigid member constituting a vehicle body. Thus, the first plate is hardly bent, and consequently, the gap 125 remains.
If roller hemming is performed until the gap 125 remains, troubles defects of a finished shape, such as wrinkling or ruffling, are caused. Since the gap 125 is not allowable, if a clamping force is increased for the purpose of correcting the gap 125, a pressing flaw will occur in the wheel arch 121, or the wheel arch 121 will deform.
From the above reason, the roller hemming using the conventional hemming die 101 cannot be applied to rigid members, such as a white body.